1. Field of the Invention
The present invention relates to a driving circuit for an LED backlight system, and more particularly, to a driving circuit driving the LED backlight system by a constant current source.
2. Description of the Prior Art
Since a liquid crystal display (LCD) can not illuminate, it must rely on a backlight system for providing an adequate and uniform light source to normally display images. Conventionally, Cold Cathode Fluorescent Lamps (CCFLs) have become the primary light source of the LCD due to high brightness and low cost. However, the CCFLs, containing mercurate, have been replaced by light emitting diodes (LEDs), as known for high color temperature, compact size, power saving, and rapid response, to meet consumer's demands for color, size, and lifetime, since the eco-awareness has been considered lately.
Generally, the driving approach of the LEDs can be categorized into two types, voltage driving and current driving to be exact. Please refer to FIG. 1, which is a schematic diagram of an LED backlight system 10 when the voltage driving is exploited. As shown in FIG. 1, the LED backlight system consists of LEDs D1˜Dn connected in parallel, and controls the current level through each of the LEDs by using current limiting resistors S1˜Sn, so as to control the brightness of each of the LEDs. Nevertheless, the use of such driving approach makes the current through each of the LEDs liable to diverge due to differences between the current limiting resistors and the forward bias of the LEDs, and thus leads to un-uniform distribution of the brightness for the backlight system.
To improve the un-uniform distribution, the backlight system changes ways to connect the LEDs from parallel to series, as shown in FIG. 2. Not only are the currents through each of the LEDs the same, but also power usage efficiency has been boosted by avoiding the use of the current limiting resistors. Yet the forward bias of the LED varies with temperature, the current through an LED string rises up when the temperature increases, causing that the brightness of the LEDs varies with the temperature.
Thus, in the prior art, the backlight system usually exploits the current driving to drive the LED strings, such that the current through each of the LEDs is equal and the current can be prevented from varying with the temperature. Thus, effectively controlling the brightness of the LEDs can be achieved, as shown in FIG. 3. In this situation, how to provide an efficient driving circuit with a constant current source becomes an important issue.